Summary:
Remove legacy logging API (`XLOGC` and friends) because it is not used anywhere
and is not fully safe being varargs-based. The code that needs to be converted
from `printf`-based logging can use fmt's `printf` API which is fully type-safe.

Reviewed By: simpkins

Differential Revision: D15036094

fbshipit-source-id: 3919e5d4eb7994ed0db4fa47bada111ab0dca505